Turning off restrictions on an iphone handed down from my son

Honestly the cleanest way with a hand-me-down is to wipe it and set it up as your own phone, rather than trying to switch the limits off one by one. Before you do that, get your son to sign his Apple ID out on it (Settings, tap his name at the top, Sign Out, needs his password). That turns off Find My too. If you skip this step the phone will ask for his Apple ID after the reset and you'll be stuck. Then Settings > General > Transfer or Reset iPhone > Erase All Content and Settings, and set it up with your own Apple ID. All the Screen Time stuff goes with the wipe. If you'd rather keep it as is, Settings > Screen Time > Turn Off Screen Time does it, but it asks for the Screen Time passcode, which whoever set it up will know.